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Stem Cell Information From:
The National Institutes of Health resource for stem cell research
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What are stem cells, and why are they important?
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Stem cells have the remarkable potential to develop into many different cell types in the
body during early life and growth. In addition, in many tissues they serve as a sort of
internal repair system, dividing essentially without limit to replenish other cells as
long as the person or animal is still alive. When a stem cell divides, each new cell has
the potential either to remain a stem cell or become another type of cell with a more
specialized function, such as a muscle cell, a red blood cell, or a brain cell.
Stem cells are distinguished from other cell types by two important characteristics.
First, they are unspecialized cells capable of renewing themselves through cell division,
sometimes after long periods of inactivity. Second, under certain physiologic or experimental
conditions, they can be induced to become tissue- or organ-specific cells with special functions.
In some organs, such as the gut and bone marrow, stem cells regularly divide to repair and
replace worn out or damaged tissues. In other organs, however, such as the pancreas and the heart,
stem cells only divide under special conditions.
Stem cells are important for living organisms for many reasons. In the 3- to 5-day-old embryo,
called a blastocyst, the inner cells give rise to the entire body of the organism, including
all of the many specialized cell types and organs such as the heart, lung, skin, sperm, eggs
and other tissues. In some adult tissues, such as bone marrow, muscle, and brain, discrete
populations of adult stem cells generate replacements for cells that are lost through normal
wear and tear, injury, or disease.
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What are the unique properties of all stem cells?
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Stem cells differ from other kinds of cells in the body. All stem cells - regardless of their
source - have three general properties: they are capable of dividing and renewing themselves
for long periods; they are unspecialized; and they can give rise to specialized cell types.
1) Stem cells are capable of dividing and renewing themselves for long periods.
Unlike muscle cells, blood cells, or nerve cells which do not normally replicate themselves -
stem cells may replicate many times, or proliferate. A starting population of stem cells that
proliferates for many months in the laboratory can yield millions of cells. If the resulting cells
continue to be unspecialized, like the parent stem cells, the cells are said to be capable of long -
term self-renewal.
2)Stem cells are unspecialized.
One of the fundamental properties of a stem cell is that it does not have any tissue -
specific structures that allow it to perform specialized functions. For example, a stem cell
cannot work with its neighbors to pump blood through the body (like a heart muscle cell),
and it cannot carry oxygen molecules through the bloodstream (like a red blood cell).
However, unspecialized stem cells can give rise to specialized cells, including heart muscle
cells, blood cells, or nerve cells.
3) Stem cells can give rise to specialized cells.
When unspecialized stem cells give rise to specialized cells, the process is called differentiation.
While differentiating, the cell usually goes through several stages, becoming more specialized
at each step. Scientists are just beginning to understand the signals inside and outside cells
that trigger each stem of the differentiation process. The internal signals are controlled by a
cell's genes, which are interspersed across long strands of DNA, and carry coded instructions
for all cellular structures and functions. The external signals for cell differentiation include
chemicals secreted by other cells, physical contact with neighboring cells, and certain molecules
in the microenvironment.
Adult stem cells typically generate the cell types of the tissue in which they reside.
For example, a blood-forming adult stem cell in the bone marrow normally gives rise to the many
types of blood cells. It is generally accepted that a blood-forming cell in the bone marrow - which
is called a hematopoietic stem cell - cannot give rise to the cells of a very different tissue,
such as nerve cells in the brain. Experiments over the last several years have purported to show
that stem cells from one tissue may give rise to cell types of a completely different tissue.
This remains an area of great debate within the research community. This controversy demonstrates
the challenges of studying adult stem cells and suggests that additional research using adult stem
cells is necessary to understand their full potential as future therapies.
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What are adult stem cells?
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An adult stem cell is thought to be an undifferentiated cell, found among differentiated cells in a
tissue or organ that can renew itself and can differentiate to yield some or all of the major specialized
cell types of the tissue or organ. The primary roles of adult stem cells in a living organism are to
maintain and repair the tissue in which they are found. Scientists also use the term somatic stem cell
instead of adult stem cell, where somatic refers to cells of the body (not the germ cells, sperm or eggs).
Unlike embryonic stem cells, which are defined by their origin (cells from the preimplantation - stage
embryo), the origin of adult stem cells in some mature tissues is still under investigation.
Research on adult stem cells has generated a great deal of excitement. Scientists have found
adult stem cells in many more tissues than they once thought possible. This finding has led
researchers and clinicians to ask whether adult stem cells could be used for transplants.
In fact, adult hematopoietic, or blood-forming, stem cells from bone marrow have been used in
transplants for 40 years. Scientists now have evidence that stem cells exist in the brain and the heart.
If the differentiation of adult stem cells can be controlled in the laboratory, these cells may become
the basis of transplantation-based therapies.
Adult stem cells have been identified in many organs and tissues, including brain, bone marrow,
peripheral blood, blood vessels, skeletal muscle, skin, teeth, heart, gut, liver, ovarian epithelium,
and testis. They are thought to reside in a specific area of each tissue (called a "stem cell niche").
In many tissues, current evidence suggests that some types of stem cells are pericytes, cells that
compose the outermost layer of small blood vessels. Stem cells may remain quiescent (non-dividing)
for long periods of time until they are activated by a normal need for more cells to maintain tissues,
or by disease or tissue injury.
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